Halo 3 has a cool new feature that lets you track your Halo stats on Bungies website. There's no need to sign up, all you have to do is go to Bungie's website and plug in your gamertag. It tracks all sorts of thing like what your favorite weapons are, how many kills you have, and how many times you have been killed. Another neat feature is the ability to capture live video clips and post them on your profile through your 360.
http://www.bungie.net/Stats/Halo3/Default.aspx?player=JohnRambo101
http://www.bungie.net/Stats/Halo3/Default.aspx?player=JohnRambo101